Method for making a superconducting field-effect transistor with inverted MISFET structure

ABSTRACT

This field-effect transistor comprises a conductive substrate (2) serving as the gate electrode, an insulating barrier layer (3), and a superconducting channel layer (1) on top of the barrier layer (3). The superconductor layer (1) carries a pair of mutually spaced electrodes (4, 5) forming source and drain, respectively. The substrate is provided with an appropriate gate contact (6). 
     The substrate (2) consists of a material belonging to the same crystallographic family as the barrier layer (3). In a preferred embodiment, the substrate (2) is niobium-doped strontium titanate, the barrier layer (3) is undoped strontium titanate, and the superconductor (1) is a thin film of a material having a lattice constant at least approximately similar to the one of the materials of the substrate (2) and barrier (3) layers. A preferred material of this type is YBa 2  Cu 3  O 7- δ, where 0 ¢δ≦0.5. 
     While the preferred embodiment of the present invention has been herein described, numerous modifications, changes and improvements will occur to those skilled in the art without departing from the spirit and scope of the present invention.

DESCRIPTION

This invention relates to field-effect transistors having a currentchannel consisting of high-T_(c) superconducting material theconductivity of which can be reproducibly influenced by an electricalfield, and whose structure is that of an inverted(Metal-Insulator-Superconductor) MISFET in the sense that the substrateis used as the gate electrode. The invention further relates to a methodfor making superconducting field-effect transistors with inverted MISFETstructure.

BACKGROUND OF THE INVENTION

For several decades, the electronics industry has made enormous effortsto shrink the size of electronic components and circuits with the aim ofincreasing the speed of operation and of reducing the powerconsumption/dissipation. These efforts have led to the development ofintegrated circuits and multi-layer ceramic devices which, in a volumeof a few cubic millimeters, contain many thousands of transistors andother circuit components. These devices have very high operating speedsowing to the shortened distances the electrons need to travel inside ofthem. All of the modern circuits use advanced semiconductor materials,such as silicon and gallium arsenide, for example.

The discovery by Bednorz and Mueller (Z. Phys., B64 (1986) p. 189) of anew class of superconductor materials has of course opened anotheravenue to even lower power consumption and caused a worldwide search forpossible applications of these materials in electronic circuitry. Anumber of studies on the electric field-effect in copper oxide compoundshave been reported (for example by U. Kabasawa et al. in Japanese Journ.of Appl. Phys. 29 L86, 1990), but so far only minor field-effects inhigh-T_(c) superconductors have been found. However, EP-A-0 324 044already describes a three-terminal field-effect device with asuperconducting channel in which electric fields are used to control thetransport properties of channel layers consisting of high-T_(c)superconductor materials. While this seemed to be a promising approach,growth studies of such devices have shown that in the suggestedconfiguration the ultrathin superconducting layers readily degradeduring deposition of insulator layer and top electrode.

In accordance with the present invention, this drawback is avoidedthrough deposition of the superconducting film after the insulatinglayer, and locating the gate electrode underneath the insulator and thehigh-T_(c) film. Still in accordance with the present invention, aconducting substrate is used as the gate electrode, and to facilitatethe growth of preferably perfect crystals, substrate and insulator arechosen from the same crystallographic family of materials, that is, thelattice constants of the materials chosen to at least approximatelymatch. For example, electrically conducting Nb-doped SrTiO₃ is used forthe substrate, and undoped SrTiO₃ is used for the insulator layer.

The use of niobium-doped strontium titanate Nb: SrTiO₃ in a high-T_(c)superconductor structure was described by H. Hasegawa et al. in theirpaper "Contact between High-T_(c) Superconductor and SemiconductingNiobium-Doped SrTiO₃ ", Japanese Journ. of Appl. Phys., Vol. 28, No. 12,December 1988, pp. L 2210-L 2212, and in their EP-A-0 371 462. Thesereferences describe a diode structure where a superconducting film isdeposited onto an Nb-doped SrTiO₃ substrate. The authors of thesereferences are only interested in measuring the rectifying propertiesand the resistance in forward and reverse directions. They "demonstratedthat there are unknown interfacial layers between the two materials", aproblem that the present invention elegantly overcomes.

This invention is based on experimental evidence for a significantelectric field-effect recently discovered to exist in thinsuperconducting films. These experiments were performed with materialsof the copper oxide class of superconductors, in particular YBa₂ Cu₃O₇₋δ. Thin films of superconducting YBa₂ Cu₃ O₇₋δ are already known fromEP-A-0 293 836. Epitaxial growth of YBa₂ Cu₃ O₇₋δ is described in EP-A-0329 103. For the purposes of the present invention, the value of "δ"shall be considered to be close to zero (preferred), but it can be aslarge as 0.5. Those skilled in the art of high-T_(c) superconductormaterials will appreciate that many other materials in that class willbe equally suited for the field-effect transistor structures of theMISFET type, as herein suggested. Also, other methods for depositingfilms of high-T_(c) materials and of SrTiO₃ are known in the art, suchas laser evaporation, electron beam evaporation and molecular beamepitaxy.

While the acronym "MISFET" is usually employed to characterizeMetal-Insulator-Semiconductor Field-Effect Transistor structures, thisterm will in the following description be maintained for describing ananalogous structure, although the embodiments of the present inventionto be described will use different materials, viz. electricallyconducting Nb-doped SrTiO₃ in place of the Metal, and a superconductorinstead of the Semiconductor.

MISFET-type structures have been developed in accordance with thepresent invention which allow the application of electric fields largerthan 10⁷ V/cm across insulating SrTiO₃ barriers on ultrathin epitaxiallygrown YBa₂ Cu₃ O₇₋δ channel layers. Epitaxial growth of YBa₂ Cu₃ O₇₋δrf-magnetron sputtering is described in EP-A-0 343 649. In thesestructures, the normal-state resistivity and the density of freecarriers in the YBa₂ Cu₃ O₇₋δ films can be modified substantially withgate voltages of on the order of 50 V.

Shortly after the discovery of the high-T_(c) superconductor materials,Bednorz et al. in their above-cited EP-A-0 324 044 predicted ontheoretical grounds that high-T_(c) superconductor materials may bear anelectric field-effect which is much larger than that in low-T_(c)superconductor materials: The length scale by which electrostatic fieldsare screened in conducting materials is given by the sum L_(D) +L_(DZ)of the Debye length L_(D) =(ε_(o) ε_(r) kT/q² n)^(1/2) and the width ofeventual depletion zones L_(DZ) =N/n. Here, γ_(o) and ε_(r) are thedielectric constants of the vacuum and of the conducting material,respectively, k is the Boltzmann constant, T is the absolutetemperature, q is the elementary charge, n is the density of mobilecarriers, and N the induced areal carrier density. Because of their highcarrier density, low-T_(c) superconductors usually screen electricfields so well that the fields only have a minor influence on materialsproperties. To attenuate the screening, recent experiments on theelectric field-effect in low-T_(c) superconductors have focused oncompounds with exceptionally low carrier density, like doped SrTiO₃,with niobium as the dopant, for example.

In high-T_(c) superconductor compounds, larger field-effects areexpected owing to their intrinsically low carrier concentration andbecause of their small coherence length. The low carrier concentrationof about 3 . . . 5×10²¹ /cm³ leads to screening lengths in the range oftenths of nanometers, and the small coherence lengths allow thefabrication of ultrathin layers with respectable critical temperatures.Superconducting films as thin as 1 . . . 2 nm have already been grown;electric fields can penetrate such films to a considerable extent.

OBJECTS OF THE INVENTION

It is an object of the present invention to minimize degradation effectsin superconducting field-effect transistors with an inverted MISFETstructure.

It is a further object of the invention to provide superconductingfield-effect transistors whose inverted MISFET structure permits thedeposition of the superconducting channel layer after the deposition ofthe insulating barrier layer.

It is still another object of the invention to provide materials for thesubstrate and insulator layer which have at least approximately the samelattice constants so as to facilitate crystal perfection.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of two embodiments of the transistor and of the inventive methodwill hereafter be explained by way of example, with reference to theattached drawings in which:

FIG. 1 is a schematic diagram of a first embodiment of a field-effecttransistor in accordance with the invention;

FIG. 2 is a schematic diagram of a second embodiment of an inventivefield-effect transistor;

FIG. 3 shows the I_(G) /V_(G) characteristics of the field-effecttransistor of FIG. 1;

FIG. 4 is a diagram showing the dependence of the changes of the channelresistivity as a function of the gate voltage V_(G) ;

FIG. 5 shows the dependence of the channel resistivity as a function ofthe absolute temperature.

TECHNICAL DESCRIPTION OF THE PREFERRED EMBODIMENT

To apply large electric fields on thin films of superconducting YBa₂ Cu₃O₇, in accordance with the invention, an inverted MISFET-structure asshown in FIG. 1 is used. In this structure, a superconducting film 1 ofthe thickness s is separated from a gate electrode 2 by an insulatinglayer 3 of the thickness t. Besides the thickness of the superconductor,the resistivity rho₁ and the breakthrough field strength E_(BD) of theinsulator are crucial parameters. The required values for E_(BD) and forrho₁ can be simply estimated, if space charge effects are neglected: Toinduce a surface charge density in superconducting film 1 whichcorresponds to the unperturbed density n of mobile charge carriers, thecapacitor consisting of gate electrode 2 and superconductor film 1 hasto be biased with a voltage ##EQU1## where ε_(i) is the dielectricconstant of the insulator 3. Equation (1) implies that to modulate thecarrier density in high-T_(C) superconductors substantially, the productε_(i) ×E_(BD) of the dielectric constant ε_(i) and the breakdown fieldstrength E_(BD) has to be of the order of 10⁸ V/cm. (For comparison,SiO₂ has an ε_(i) ×E_(BD) product of 4×10⁷ V/cm at room temperature).

In addition, the normal-state resistivity of the insulator has to besufficiently high to avoid leakage currents which result in an inputloss V_(G) ×I_(G). For a typical case of I_(G) <I_(DS) /100 and I_(DS)=10 μA, and an area of the gate electrode 2 of 1 mm², the resistivitymust be higher than 10¹⁴ Ωcm/ε_(i) at operating temperature.

In view of these requirements, insulating layers with high dielectricconstants are recommended. Therefore, and for its good compatibilitywith the growth of YBa₂ Cu₃ O₇, SrTiO₃ is selected as barrier materialfor insulating layer 3. The compatibility of YBa₂ Cu₃ O₇, with SrTiO₃has already been pointed out in EP-A-0 299 870 and EP-A-0 301 646, andthe use of a buffer layer of oriented polycrystalline SrTiO₃ has beendescribed in EP-A-0 324 220 and EP-A-0 341 788. The method recommendedfor fabricating the MISFET structure of FIG. 1 with a SrTiO₃ barrierlayer 3 involves the following steps:

1. A gate electrode 2 is provided in the form of a (conductive) n-type{100}-oriented Nb-doped SrTiO₃ single crystal grown by a conventionalzone melting technique. The doping factor is between 10⁻³ and 5%niobium, preferably at 0.05% Nb. This gate electrode 2 is used as thesubstrate for all further depositions.

It should be pointed out that, while a single-crystal substrate ispreferred, a polycrystalline or amorphous substrate may be used as well.Also, dopants other than niobium may be used to render the SrTiO₃conducting. One example is an oxygen deficit.

2. On top of the substrate 2, a {100}-oriented insulating layer 3 ofSrTiO₃ is epitaxially grown by reactive rf-magnetron sputtering at 6,7Pa in an O sub 2/Ar atmosphere at 650° C. (temperature of the sampleholder). The thickness of this layer can be in the range of 1 to 1000nm.

3. Without breaking vacuum, a superconducting film 1 of YBa₂ Cu₃ O₇₋δ isepitaxially grown on top of the insulating SrTiO₃ layer 3 by hollowcathode magnetron sputtering, wherein the value of "δ" is preferablyequal to zero, but can be as large as 0.5. The thickness of thesuperconductor layer can be in the range of 1 to 1000 nm.

4. Metal pads, for example gold pads 4 and 5, are then sputtered ontothe YBa₂ Cu₃ O₇₋δ top layer 1 to form source and drain contacts,respectively.

5. Finally, a gate contact 6 is provided by diffusing silver into theNb-doped SrTiO₃ gate/substrate 2.

FIG. 2 shows a slightly different (but more truly MISFET) structure. Thepreferred way to manufacture this structure involves the followingsteps:

1. A {100}-oriented SrTiO₃ layer 7 is provided as an insulator which ispolished down to a thickness of 20 . . . 30 μm.

2. On top of the thinned insulator 7, a YBa₂ Cu₃ O₇₋δ film 8 issputtered, wherein the value of "δ" is preferably equal to zero, but canbe up to 0.5.

3. Gold pads 9 and 10 are provided on top of the superconductor layer 8to form source and drain contacts, respectively.

4. On the back side of the thinned insulator, a conducting gateelectrode 11 in the form of a metal layer, a gold layer, for example, isdeposited. It bears an appropriate contact 12.

5. Optionally, gate electrode 11 may be supported on an insulatingsubstrate 13, be it for stability, as indicated in FIG. 2.

FIG. 3 shows a typical characteristic of the gate current I_(G) throughthe insulating layer 3 as a function of the applied gate voltage V_(G)for a device in accordance with FIG. 1.

The measured characteristic is the one expected for a pin-junction, thesuperconductor and the substrate being the p and n electrodes,respectively. In one sample studied, the insulating barrier had aresistivity of 4×10¹³ Ωcm at a forward bias of 3 V, and of 4×10¹⁴ Ωcm ata reverse bias of 20 V. Breakdown field strengths at room temperature of5×10⁵ V/cm and of 1.5×10⁷ V/cm were obtained in the forward and reversedirections, respectively. The capacitance of this sample was 2×10⁻⁷F/cm² at room temperature. This value corresponds to a relatively lowε_(i) =8 (t=40 nm). This low dielectric constant may be caused by aninsulating surface layer on the Nb- doped SrTiO₃ substrate, which wasobserved in agreement with a report by Hasegawa et al. in Japan. Journ.of Appl. Phys., 28 L2210 (1989). The layer has a breakdown voltage ofabout 2 V. Nevertheless, the ε_(i) E_(BD) products of the SrTiO₃ barrierlayers under reverse bias were about 10⁸ V/cm, which is the limit of thevalues required by Eq. (1).

The influence of the gate voltage V_(G) on the channel resistance R_(DS)of a sample that consists of 10 nm thick YBa₂ Cu₃ O₇₋δ film on a 40 nmthick SrTiO₃ barrier layer is shown in FIG. 4. Obvious from the diagramis an approximately linear dependence of the measured normal-stateresistivity on the gate voltage, and that the effect on resisitivitychanges sign when the gate voltage is reversed. The measured polarity ofthe voltage induced resistance change agrees with the theoreticalexpection: YBa₂ Cu₃ O₇₋δ is a p-type conductor, hence a positive voltageV_(G) at the gate electrode depletes the free carrier concentration inthe channel and, therefore, increases the channel resistance R_(DS),whereas a negative voltage V_(G) at the gate electrode enhances the freecarrier concentration in the channel and, therefore, decreases thechannel resistance R_(DS).

The value of the measured channel resistance R_(DS) (V_(G)) agrees wellwith the theoretical prediction: Applying 30 V to the sample that wasused to generate FIG. 4 and which has a capacitance of 2×10⁻⁷ F/cm²,induces a change in the electron density on the electrodes of 2×10¹³/cm². On the other hand, YBa₂ Cu₃ O₇₋δ has a carrier density of about3-5×10²¹ /cm³, and this corresponds to an areal density of mobile holesin the 10 nm thick channel layer of 3-5×10¹⁵ /cm². This means that,within experimental error, at any temperature a change of the freecarrier density in the channel results in a corresponding change inR_(DS).

The temperature dependence of the resistivity R_(DS) (T) of the YBa₂ Cu₃O₇₋δ is shown in FIG. 5. The sample is typical for all devices of thetype shown in FIG. 1.

The temperature dependence of the voltage-induced variation of thechannel resistance change Delta R_(DS) /R_(DS) (V_(G) T) for this sampleis depicted in FIG. 4. As shown by this figure, within experimentalscatter, the fractional change of the channel resistance Delta R_(DS)/R_(DS) is almost constant as a function of temperature. Atemperature-independent Delta R_(DS) /R_(DS) (V_(G) T) ratio is observeddown to T_(C) (R_(DS) =0). The change of the channel resistance inducedby the gate voltage V_(G) corresponds to a change of the R_(DS) (T)characteristic at midpoint T_(C) of 50 mK for V_(G) =18 V.

From the measurements taken with several sample embodiments of thefield-effect transistor in accordance with the invention, it has beendetermined that the operating gate voltage V_(G) should be in the rangebetween 0.1 and 50 V, the thickness s of the superconducting film shouldbe in the range between 1 and 30 nm, and the thickness of the insulatinglayer should be in the range between 0.3 and 100 nm.

To make sure that the structures prepared in accordance with the presentinvention indeed perform as expected, i.e. that the current flow in thechannel can actually be controlled by the electrical field-effect,spot-checking measurements have been made as follows:

1. Measurement of R_(DS) (V_(G)) on samples that had barrier layerresistances which were lower by a factor of 500 (at 20 V) than thesample yielding the curves in FIG. 4. This measurement showed the sameR_(DS) (V_(G)) characteristics, demonstrating that the observed effectis not caused by the finite gate current I_(G).

2. To elucidate whether V_(G) primarily affects the channel resistanceR_(DS) or whether the effect is based on a change of V_(DS), R_(DS) wasmeasured for different channel currents I_(DS). Even if I_(DS) is variedby four orders of magnitude, an applied gate voltage results in a changeof the channel resistance R_(DS) R and does not induce significantvoltages in the channel layer.

MISFET-type heterostructures consisting of YBa₂ Cu₃ O₇₋δ /SrTiO₃multilayers have been developed which allow the application of electricfields larger than 10⁷ V/cm to superconducting films of YBa₂ Cu₃ O₇₋δ.In these devices, electric field-effects generate changes in the channelresistance. The YBa₂ Cu₃ O₇₋δ films have a preferred thickness on theorder of 10 nm and are operated with gate voltages of about 30 Volts.The channel resistivity changes can be attributed to equally strongchanges of the carrier density in the high-T_(c) superconductor.

We claim:
 1. A method for making a field-effect transistor having anelectrical field-controlled current channel and gate, source and drainelectrodes comprising the steps of:a) providing a gate electrode (2) ofa conductive n-type {100} oriented Nb-doped SrTiO₃ single crystal thathas a doping factor between 0.001% and 10% niobium, said gate electrodeserving as a substrate; b) depositing on top of said gate electrode (2)a {100}-oriented insulating layer (3) of SrTiO₃ ; c) depositing on topof said insulating SrTiO₃ layer (3), a superconducting thin film (1) ofYBa₂ Cu₃ O₇₋δ, wherein 0≦δ≦0.5; d) depositing onto said YBa₂ Cu₃ O₇₋δlayer (1) gold pads (4,5) to form source and drain contacts,respectively; and e) depositing a gate contact (6) on the Nb-dopedSrTiO₃ gate/substrate (2).
 2. A method as in claim 1, wherein said layer(3) of SrTiO₃ is epitaxially grown by reactive rf-magnetron sputteringat 6.7 Pa in an O₂ /Ar atmosphere at 400° to 900° C.
 3. A method as inclaim 1, wherein said superconducting film (1) of YBa₂ Cu₃ O₇₋δ isepitaxially grown on top of the insulating SrTiO₃ layer (3).
 4. A methodas in claim 1, wherein said gate contact (6) is provided by diffusingsilver into the Nb-doped SrTiO₃ gate/substrate (2).
 5. The method as inclaim 1 wherein said doping factor is 0.05% Nb.